Pistol caliber carbines will be in the 29-36 inch range so it should be easier to move around a house than it is with a short barreled shot gun, most HD shot guns,excluding pistol grip only(a really bad idea)types are in the 40 inch area.
If she likes rifles let her look and the carbines and see if that is right for her.
My wife's favorite rifle is my Uzi carbine with the 16 inch barrel.